The present disclosure relates to the use of electrically conductive nanoparticles in combination with non-electrically conductive micron-sized fillers to enhance the thermal conductivity of polymer matrices.
Many electrical components generate heat during periods of operation. If this heat is not removed from the electrical component in an efficient manner, it will build up. Malfunction or permanent damage to the electrical components may then result. Therefore, thermal management techniques are often implemented within electrical circuits and systems to facilitate heat removal during periods of operation.
Thermal management techniques often involve the use of some form of heat sink to conduct heat away from high temperature areas in an electrical system. A heat sink is a structure formed from a high thermal conductivity material (e.g., typically a metal) that is mechanically coupled to an electrical component to aid in heat removal. In a relatively simple form, a heat sink can include a piece of metal (e.g., aluminum or copper) that is in contact with the electrical circuit during operation. Heat from the electrical circuit flows into the heat sink through the mechanical interface between the units.
In a typical electrical component, a heat sink is mechanically coupled to the heat producing component during operation by positioning a flat surface of the heat sink against a flat surface of the electrical component and holding the heat sink in place using some form of adhesive or fastener. As can be appreciated, the surface of the heat sink and the surface of the component will rarely be perfectly planar or smooth, so air gaps will generally exist between the surfaces. As is generally well known, the existence of air gaps between two opposing surfaces reduces the ability to transfer heat through the interface between the surfaces. Thus, these air gaps reduce the effectiveness and value of the heat sink as a thermal management device. To address this problem, polymeric compositions referred to as thermal interface materials or TIMs have been developed for placement between the heat transfer surfaces to decrease the thermal resistance therebetween. Many TIM applications, including those used with electrical components, require the TIM to be electrically insulating. Also in many TIM applications, the TIM must be sufficiently compliant to provide mechanical isolation of the heat generating component and the heat sink in those cases where the Coefficient of Thermal Expansion (CTE) of the component is significantly different (higher or lower) than that of the heat sink. The thickness and the material composition of the TIM can be restricted by the need for mechanical compliance. The minimum thickness of the TIM is determined by the degree of surface planarity on both the component and the heat sink.
The bulk thermal conductivity of current thermal interface materials is largely limited by the low thermal conductivity of polymer matrices (˜0.2 W/m-K for polymers typically found in TIMs). In order to enhance their thermal conductivity, many TIM materials are filled with particles that have a higher thermal conductivity (>10 W/m-K). By some estimates (“Thermally Conductive Polymer Compositions,” D. M. Bigg., Polymer Composites, June 1986, Vol. 7, No. 3), the maximum bulk thermal conductivity attainable by electrically insulating polymer composites is only 20-30 times that of the base polymer matrices. This number changes little regardless of the filler type, once the thermal conductivity of the filler exceeds 100 times that of the base polymer matrix. Consequently, the thermal conductivity of polymeric materials is low compared to the thermal conductivity of the heat sink, resulting in an inefficient transfer of heat from the heat producing component to the heat sink. In addition, effective heat transfer capability is further reduced by interfacial imperfections due to 1) micro or nanovoids, and 2) a filler-depleted layer caused by filler settlement or the inability of micron-sized filler to penetrate into surface irregularities that are smaller than the filler size.
While metals and other electrically conductive materials frequently are thermally conductive materials, for non-electrically conductive applications these higher performance materials either cannot be used in the TIM or they must be coated with a non-electrically conductive material, thereby adding cost, reducing thermal performance and potentially risking having openings in the non-electrically conductive coatings which could cause an electrical short. Thus, in most cases non-electrically conductive materials must be used, thereby limiting material choice and generally limiting thermal conductivity.
In addition to fillers, attempts to enhance the thermal conductivity of TIMs have included the use of nanoparticle materials. For example, U.S. patent application Ser. No. 10/426,485 discloses the use of non-electrically conductive nanoparticles in a polymer matrix to improve the thermal conductivity of a TIM system. However, for the reasons noted above, the choice of materials for use as the nanoparticles is limited.
In other electronic component applications where the heat generated during operation is lower, alternate thermal cooling approaches are utilized. These components often go into portable electronics such as lap top personal computers, cellular phones, digital assistants and electronic cameras. These components are often mounted onto a printed circuit board composed of a polymer material by means of an array of solder spheres. Reliability concerns with the integrity of the solder joints during normal thermal cycling due to environmental changes and due to power cycling, has lead to the use of resin underfill material that fills the gaps between the solder spheres under the electrical component. In many applications the prime thermal cooling path is from the component into the printed circuit board. Without any underfill or with an underfill that is a poor thermal conductor, the only thermal path from the component to the board is through the solder. The thermal performance can be improved by adding thermally conductive fillers to the underfill resin. In this application area, the resin can not be electrically conductive as it would short out the component I/O pads. Therefore underfill resins are limited to the use of electrically non-conducting fillers. As in the case of the TIM materials, this limits the thermal conductivity achievable. A need therefore exists for improved compositions to effectively transfer heat between a printed circuit board and a heat producing component in non-electrically conducting underfill materials.
A need therefore exists for improved compositions to effectively transfer heat between a heat sink and a heat producing component, especially in non-electrically conductive applications.